Quantisers in general are well-known. A quantiser receives an input signal and produces an output signal that indicates into which, of a set of amplitude ranges, the input signal falls. A typical application of a simple quantiser is in an analogue-to-digital converter in which the sizes of the ranges, or “quantisation steps”, are set by the precision of the output digital signal and the acceptable input signal range. However, the quantisation steps need not all be the same size. For example, an analogue-to-digital converter can be used as a speech compressor by making the quantisation steps smaller for large input signals than for small input signals.